Power Transformer

[mokruxa]

if the requirements for the power transformer are the following:
cooling: ONAN
continuous power rating with 55 deg.rise over 40 deg. ambient= 15MVA

Overload Power Ratings: Transformers shall be capable to supply the following load cycle within the specified temperature rise at 40°C ambient temperature:
1) 100% of continuous rating indefinitely 55 degrees C rise
2) Followed by 150% of continuous rating for two hours 65 degrees C rise
3) Followed by 300% of continuous rating for five minutes 75 degrees C rise

How would be transformer's power rating indicated as ?

Is it 55/65/75 which correspond to 15/25/45MVA ? Am I correct. Thanks

 

[Julius Right]

In my opinion, the answer is more complicated. It depends on a lot of factors. See for instance IEC 60076-7 ch.8 Determination of temperatures and Annex B.:weeping:

 

[Julius Right]

According to a rough calculation the final temperature rise [if for 100% it is 55oC] will be 150oC for 150% and 640oC for 300%.
The short time loading does not reach the final temperature.

 

[mike_kilroy]

if the requirements for the power transformer are the following:
cooling: ONAN
continuous power rating with 55 deg.rise over 40 deg. ambient= 15MVA

Overload Power Ratings: Transformers shall be capable to supply the following load cycle within the specified temperature rise at 40°C ambient temperature:
1) 100% of continuous rating indefinitely 55 degrees C rise
2) Followed by 150% of continuous rating for two hours 65 degrees C rise
3) Followed by 300% of continuous rating for five minutes 75 degrees C rise

How would be transformer's power rating indicated as ?

Is it 55/65/75 which correspond to 15/25/45MVA ? Am I correct. Thanks

no.

1) yes. starting at 40C temperature, after the transformer's thermal time constant, the transformer will be at 63% of 55C rise plus the 40C ambient. after 3 time constants, it will rise 97% of 55C and thus be close to max continuous rated temp of 95C.
2) not "followed by:" rather, starting at 40C temp, applying 150% load for 2 hours will make the temp rise 65C for total transformer temp of 105C
3) not "followed by:" rather, starting at 40C temp, applying 300% load for 5 minutes will make the temp rise 75C for total transformer temp of 115C